The present invention relates to a bus system which permits data communication with a high efficiency between an external resource and a processing unit, which is suitable for a processing unit such as a controlling computer for which a high real time processing ability is required.
In the processor system described in the above-mentioned application, machine state controllers for generating access control signals to external resources are distributedly arranged in a group of multi-divided external resources, and the machine state controllers are operated in parallel in synchronism with a plurality of system basic clocks which have a most advanced phase are adjusted in phase. The machine state controllers pre-decode an internal state signal of a CPU to locally generate a bus cycle prior to the start of the access by the CPU to conduct the pre-access to the resource so that a delay in an input/output buffer of the machine state controller is avoided. By locally generating the bus cycle prior to the start of the access by the CPU and conducting the pre-access to resource, the high speed operation and assurance of access time are attained. For the control items which require various kinds of data processing and the control items whose contents of processing and data to be handled dynamically change, the addresses at which the data to be processed are present randomly change and the execution unit in the processing unit cannot acquire necessary data in each machine cycle or bus cycle.
The above problem is not very serious in a normal computer utilization environment in which vector type numeric processing in a static environment is the principal processing, but it is particularly serious in a utilization environment such as a controlling computer in which dynamic problems are to be processed in real time.
In a processing system having a complex processing algorithm and a close linkage to an external environment such as a large scale CAD system, the random data processing and the frequency of access to the external resource significantly increase, and real time performance comparable to that of the dynamic processing system is required. It is considered that use of such a system will increase in future and the above problem will become serious even in the general utilization environment.
In a control application, since the feedback between the resource and the processing units (a processing unit processes based on the data of the resource, the processed result is returned to the resource, another processing unit controls a control item based on the returned result and the other processing unit feeds back the control result as resource data) frequently occurs, the delay due to the feedback causes a reduction of the real time performance.